This invention relates to energy storage cells, and, more particularly, to the control during service operations of the gas pressure within pressurized energy storage cells.
Rechargeable cells or batteries are electrochemical energy storage devices for storing and retaining an electrical charge and later delivering that charge as useful power. Familiar examples of the rechargeable cell are the lead-acid cell used in automobiles, and the nickel-cadmium cell used in various portable electronic devices. Another type of cell having a greater storage capacity for its weight is the nickel oxide/pressurized hydrogen cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications. The weight of the spacecraft storage cell must be minimized while achieving the required performance level, due to the cost of lifting weight to earth orbit and beyond.
The nickel-hydrogen cell includes a series of active plate sets which store an electrical charge electrochemically and later deliver that charge as a useful current. The active plate sets are packaged within a hermetic pressure vessel that contains the plate sets and the hydrogen gas that is an essential active component of the cell. Each plate set includes a positive electrode, a negative electrode, and a separator between the two electrodes, all soaked with an electrolyte. In a typical cell, a number of plate sets are supported on a core under a compressive loading, with a gas screen between each plate set and with electrical connector leads extending to each electrode of each plate set. The gas screen provides a gas channel from the hydrogen electrode to the gas space outside the stack. A single nickel-hydrogen storage cell delivers current at about 1.3 volts, and a number of the cells are usually connected in series to produce current at the voltage required by the systems of the spacecraft.
During the operation of the nickel-hydrogen cell, hydrogen is produced within the pressure vessel. The hydrogen is intentionally produced during the charging portion of the charging/discharging cycle. Hydrogen is also undesirably produced by other mechanisms such as corrosion of the components within the cell by the electrolyte. With increasing service, the maximum hydrogen pressure within the cell could gradually rise to a value above the design limit of the pressure vessel and which could cause the pressure vessel to fail.
There is a need for an approach for avoiding potential pressure vessel failures due to hydrogen overpressures. The pressure vessel itself could be made stronger, but that would result in increased weight of the cell. The nickel precharge of the cell could be increased, but this approach would decrease the initial cell storage capacity. An electrical or electromechanical valve could be provided in the cell wall to release hydrogen, but such a valve would have to be highly reliable over extended service periods of many years and must not allow a reverse leakage of any contaminant cases into the hermetically sealed pressure vessel. No such valve is known at this time, and such a valve would add a significant amount of weight to the cell.
Accordingly, there is a need for an improved approach for the design of a storage cell in which there is the possibility of an overpressure of a gas such as hydrogen being produced during service. The present invention fulfills this need, and further provides related advantages.